1. Field of the Invention
The present invention relates generally to communications. More particularly, the present invention relates to amplifier circuitry used in communications systems.
2. Description of the Related Art
In communications, intermodulation products may be generated when processing fundamental frequencies through the amplifier circuitry of a transmission system. See FIG. 1. If not suppressed, then these intermodulation products can cause interference and distortion in the transmitted signal.
There are several existing methods of suppressing intermodulation products, such as filtering, reducing the operating efficiencies of the power amplifier, and feed-forward control systems. Each of these methods has disadvantages. Filtering in some systems, such as an EDGE modulation system, cannot easily remove third order intermodulation products (IM3) because IM3 products are very close to the primary carrier of the intended signal(s). Reducing operating efficiencies of the amplifier prevents growth of intermodulation products, but results in less amplification at a particular stage and generates more heat to be dissipated into the surrounding system.
FIG. 2 illustrates a prior art single loop feed-forward control circuit that uses a reverse diode configuration in the feedback path. The idea is to reproduce the portion of the signal clipped by the amplifier when operating at or near compression by using the reverse diode circuit. The replicated signal can then be reinserted back into the main path to pump up the primary carrier, which makes the intermodulation products look smaller. The feedback configuration will not work unless the amplifier is operating near compression. In other words, it has no effect on intermodulation products when the signal is backed away from compression. In addition, the configuration disadvantageously requires a delay line in the feedback path.
A dual loop feed-forward control circuit has a first loop which acts to null the primary carrier(s) by comparing a delayed version of the primary carrier(s) against a phase adjusted, amplified version of the primary carrier(s) having intermodulation components. See FIG. 3. The intermodulation products resulting from the comparison are delivered to the second loop. The amplitude and phase of those intermodulation products are adjusted and inserted back into the main path with a delayed version of the primary carrier in such a way as to suppress (cancel or reduce) the main amplifier's intermodulation products.
A dual loop feedback system may suppress intermodulation products, but the delay lines and multiple couplers required to align, sample and merge signals decreases the efficiency and significantly increases the size. The delay lines must be large if large bandwidths are required (>1 MHz). Efficiency of the circuit is important, but in next generation multicarrier systems, circuit size is also important.